1. Field of the Invention
The present invention relates generally to a microelectromechanical device and more particularly to a microelectromechanical device using resistive electromechanical contact.
2. Description of the Related Art
There are many microelectromechanical devices using resistive electromechanical contact between conducting elements and one example is a resistive microelectromechanical switch which switches on or off input and output terminals of a signal transmission line. Contact characteristics of a resistive microelectromechanical device affects its electrical performance such as power loss, power handling, and reliability. For a resistive switch, high contact resistance in the on-state results in large power loss and large resistive heat (Joule heat), which causes limited power handling and poor reliability. Electromechanical contact used by a microelectromechanical device transmits an electronic signal through mechanically contacting at least two conducting elements. And it is different from solid-state electronic contact, where an electronic signal is transmitted through at least two conducting elements in a solid body, used by a semiconductor device. Unlike in the solid-state electronic contact, high contact resistance and resultant large resistive heat may be generated in electromechanical contact between conducting elements with low resistivity. In general, the large resistive heat causes micro-welding problem on contacting surfaces of two conductors, and a device failure to return to on state, that is non-contact state. The micro-welding problem becomes more serious with the higher transmission power through the contact, the longer on-state time and the more on/off cycles, because of the larger resistive heat, the longer time for heat accumulation and the larger thermomechanical damage on the contacting surfaces. Accordingly, in order to improve the reliability, lifetime and power handling of microelectromechanical devices using resistive electromechanical contact, it is necessary to effectively prevent the micro-welding problem occurring on contacting surfaces of two conductors.
In general, in a microelectromechanical device using resistive electromechanical contact, a contact pad, which connects input and output terminals, is formed of gold (Au) as a signal line. This is to reduce power loss by maintaining low resistivity, e.g., −2×10−6 Ω cm, even during a post-process under an oxidation ambient. However, gold has poor thermomechanical characteristics and thus is vulnerable to the micro-welding problem, which is inevitably generated at electromechanically contacting surfaces of two conductors due to resistive heat. To solve this problem, it is suggested that the gold contact pad be coated with platinum (Pt) that has higher resistivity of −1×10−5 Ω, but better thermomechanical characteristics than gold. This suggestion will now be explained with reference to FIG. 1.
FIG. 1 is a cross-sectional view of a cantilever switch that is one of general microelectromechanical devices. Referring to FIG. 1, the cantilever switch is formed on a substrate 20, and includes an anchor 12, a contact pad 16, and a pull-down electrode 18. A cantilever 14 includes one end portion 22 connected to the anchor 12, a central portion 24 above the contact pad 16, and the other end portion 26 above the pull-down electrode 18. The cantilever 14 is formed of a platinum layer 25 and a gold layer 23. The contact pad 16 is formed of a titanium layer 36, a gold layer 38, and a platinum layer 40. The pull-down electrode 18 is formed of a titanium layer 32 and a gold layer 34. In operation, the cantilever switch of FIG. 1 is switched on when the pull-down electrode 18 is given an electrostatic charge from a power supplier 30 and thus the cantilever 14 is connected to the contact pad 16. On the contrary, the pull-down electrode 18 is not provided by an electrostatic charge, the cantilever 14 is disconnected from the contact pad 16, and thus, the cantilever switch is in the off state.
As previously mentioned, a general cantilever switch has a contact pad which is a gold layer covered with a platinum layer. However, platinum is noble metal like gold, and thus, its thermomechanical features is so poor that cannot basically prevents the occurrence of micro-welding problem.